Ts 335 and ts 337 are temperature sensitive mutants of the avian tumor virus, Rous sarcoma virus (RSV). A preliminary characterization (Linial and Mason, 1973) has revealed that these mutants contain a temperature sensitive RNA-dependant DNA polymerase (reverse transcriptase) activity. Further studies of ts 335 and ts 337 will be carried out in chick fibroblast tissue cultures to investigate: 1. Whether DNA synthesis required for viral infection includes cellular DNA synthesis or only viral directed synthesis, and the relationship of the cell cycle to viral infection. 2. The role of the reverse transcriptase activity in maintaining viral replication and cellular transformation. 3. A genetic analysis of reverse transcriptase genes. Reverse transcriptase activity is currently being used to diagnose possible RNA tumor virus involvement in human malignancies. However, the role of this enzyme in oncogenesis is not clearly defined. The model system described here, utilizing unique viral temperature sensitive mutants, will be used in order to investigate the relationship of viral enzyme and viral directed DNA synthesis to production of virus particles and cellular transformation.